The present invention relates to upstream transmission in a cable television distribution plant. More particularly, the present invention relates to a method for reducing delay distortion and for allocating frequencies for upstream transmission in a cable television distribution plant.
Conventionally, a CATV distribution plant includes cable modems (CMs), some of which are located at customers"" premises. CMs transmit and receive packets from a cable modem termination system (CMTS) that is located at a head-end. The CMTS transmits signals to CMs in the downstream bandwidth, which is approximately 52-750 MHz. An upstream transmission in a CATV distribution plant, i.e., from a customer""s premises toward the CMTS, is made difficult by delay distortion caused by diplexing filters that are needed at each bi-directional amplifier, CM and fiber node. Generally, two diplexing filters are used for each bi-directional amplifier and one diplexing filter is used for each CM and fiber node.
One approach for dealing with delay distortion is to train a preemphasis filter in a transmitter at each CM to compensate for delay distortion produced by diplexing filters in each transmission path. This approach may work well, but providing an adjustable preemphasis filter at each transmitter is expensive and the training slows the already too slow process of starting up a cable modem. Hence, a need exists for providing a simpler and less expensive solution for reducing delay distortion in upstream transmissions, thereby improving signal quality.
Generally, group delay, wherein some frequencies exhibit greater time delay than others, distorts an upstream transmission. FIG. 6 illustrates group delay caused by a bi-directional amplifier having two diplexing filters for an upstream bandwidth of approximately 5-42 MHz. The differential group delay curve illustrates the dramatic change in group delay for frequency ranges A and B, near the ends of the upstream bandwidth. For example, frequency range B, which is near the split between the upstream and downstream bandwidths, has a dramatic change in group delay. Upstream transmissions in frequency ranges A and B will have relatively large delay distortion that results in signal degradation and, possibly, data loss. The upstream bandwidth may include a frequency range other than approximately 5-42 MHz, e.g. approximately 750 MHz-1 GHz. Delay distortion, however, generally increases dramatically near the split between the upstream and downstream bandwidths, and thus presents a problem for an upstream transmission on a frequency near the split. Thus, a need exists for reducing delay distortion of upstream transmissions on frequencies near a split between the upstream and downstream bandwidths, so the upstream bandwidth can be maximally utilized.
Furthermore, for an upstream bandwidth of approximately 5-42 MHz, the lower end of the upstream bandwidth, which also suffers from increased delay distortion, is noisy. Thus, if delay distortion is reduced at the lower end of the bandwidth, noise is still a problem. On the other hand, the upper portion of the upstream bandwidth, e.g., approximately 38-42 MHz, is relatively quiet, but suffers from relatively large delay distortion. Consequently, a need exists for reducing delay distortion in the upper portion of the upstream bandwidth, near the split between the upstream and downstream bandwidths, in order to take advantage of the relatively quiet upper portion of an upstream bandwidth having a range of approximately 5-42 MHz.
Accordingly, it is an aspect of the present invention to provide simple and less expensive solutions for reducing delay distortion for upstream frequencies near the split between the upstream and downstream bandwidths.
In accordance with an aspect of the present invention a database is provided for storing the CM identification (ID), the CM type, the bi-directional amplifier type, the number of bi-directional amplifiers of each type and the fiber node type for each transmission path. Delay distortion for a transmission path can be calculated using the information stored in the database corresponding to the CM associated with the transmission path.
Also, in accordance with another aspect of the present invention, the upper portion of an upstream bandwidth may be assigned to a CM having a transmission path that causes relatively minimal delay distortion. Calculated delay distortion for a CM is compared to a predetermined threshold, and the CM is preferably assigned to the upper portion of the upstream bandwidth if the calculated delay distortion is less than the predetermined threshold. Furthermore, a fixed equalizer may be used for minimizing delay distortion for upstream transmissions.
Also, in accordance with another aspect of the present invention, the upper portion of an upstream bandwidth may be assigned to the largest number of CMs that have the same number of bi-directional amplifiers in their respective transmission paths. Calculated delay distortion for a CM is compared to a predetermined threshold, and the CM is preferably assigned to the upper portion of the upstream bandwidth if the calculated delay distortion is equal to the predetermined threshold. Furthermore, a fixed equalizer may be used for minimizing delay distortion for upstream transmissions.
Also, in accordance with another aspect of the present invention, a variable equalizer at the fiber node may be provided for minimizing delay distortion of an upstream transmission. Delay distortion for the transmission path is calculated using information from the database corresponding to the CM associated with the transmission path. The variable equalizer is set to minimize the calculated delay distortion.